In assaying urine samples for electrolytes, a common problem that complicates the assay is the wide range of ionic strengths that potentially occur in the samples. That is, the ionic strength can range from about 50 to about 400 millimolar, depending on the health of the patient. Such a 10 to 1 range tends to create errors because of the variation in activity coefficients and junction potentials that occur over that range.
Therefore, it is common practice to add a charged diluent to the urine to compress the ionic range. For example, if 4 parts of a diluent of 150 millimolar strength are added to 1 part of urine, the above-noted range of 50 to 400 is compressed as follows: Only 1/5 of the mixture is urine, so the urine's ion strength will be from 10 to 80 millimolar. Adding the 150 millimolar of the diluent produces a range of 160 to 230 millimolar, a much more acceptable and workable range.
A further common problem, particularly in certain biological liquids such as urine, has been the presence of NH.sub.4 .sym., either because of pathologies of the patient or because of sample aging. The ammonium cation is a known interferent for a number of assays. To avoid this problem, it is known to use a diluent having a pKa that is large enough to convert ammonium to ammonia. Thus, if ammonium is a potential interferent, a diluent such as Tris buffer, which has the structural formula (HO-CH.sub.2).sub.3 C-N.sup..sym. H.sub.3 Z.sup..crclbar. where Z is an acid anion, is considered to be inferior because the pKa is not greater than or equal to 9.3.
Even if the pKa is sufficiently high, conventional diluents present a further problem. Recent developments have provided an improved method for conducting potentiometric assays of serum using a pair of disposable ion-selective electrodes (hereinafter, "ISE's"). Such ISE's are mounted in a plastic frame and permit rapid processing at high through-put rates in an automated analyzer, for example the analyzer available under the trademark "Kodak EKTACHEM 400" from Eastman Kodak Co. The plastic frame and ISE's are further described in U.S. Pat. No. 4,053,381, issued on Oct. 11, 1977, and U.S. Pat. No. 4,214,968, issued on July 29, 1980. Such ISE's comprise a dried internal reference electrode comprising the residue of a solution of a salt and a hydrophilic polymeric binder in a solution for the polymer and the salt. In contact with the reference electrode, there is a hydrophobic ion-selective membrane comprising a hydrophobic binder and an ion carrier in a carrier solvent. The use of two such ISE's with a patient sample and a reference liquid having a known concentration of the ion in question provides a differential measurement. Because of the ease and speed with which such assays of serum are accomplished, it is desirable that other types of liquids be assayed on such analyzers. The use of the afore-described ISE's and analyzer with other liquids besides serum is becoming an accepted practice. However, when assaying liquids such as urine, the presence of a cationic diluent as noted above introduces the potential of an interferent, as follows: Certain ISE's of the afore-mentioned patent, particularly those used to assay for Na.sup..sym., feature an ionophore that is not always selective enough to preclude detection of certain interferents. As a result, the cations of many conventional diluents tend to act as interferents when testing, e.g., for Na.sup..sym.. The interference shows up in the slope of the calibration curve (millivolts versus the log of the analyte concentration) in that the slope is reduced compared to the Nernstian ideal (60 millivolts/decade). Reductions in slope are caused by a departure from linearity in the calibration curve, primarily at the low ion concentrations. If the slope is reduced to 55 or less, the error at low ion concentrations is so large that the assay is unacceptable. Thus, for example, the diluent described in Anal. Chem., Vol. 38, pp. 1951-1954 (1966), namely diethylamine, acetate salt, produces a slope of less than 55 when used to assay liquids using the above-noted ISE's.
Therefore, prior to this invention there has existed a need for a charged diluent, useful for example in assaying urine electrolytes, that does not act as an interferent for Na.sup..sym. when using an ISE especially of the type described in the aforesaid U.S. patents, while at the same time reducing the ammonium interference.